The cell surface receptor for cholera toxin is the ganglioside GM1. This has been shown in all vertebrate cells examined to date including fat cells which are very sensitive to the toxin but have only trace amounts of GM1. Cholera toxin is multivalent and binding to more than one GM1 receptor on the cell surface is a necessary step for the subsequent activation of adenylate cyclase by the toxin. Although there are similarities between cholera toxin and the hormones thyrotropin and gonadotropin, which also activate adenylate cyclase, the specific determinants for hormone binding appear to reside on membrane proteins in the respective target cells. The interaction of these hormones with their receptors may be influenced by membrane phospholipids and the internalization of the hormonal signal may be mediated by membrane gangliosides. When avian myoblasts fuse in culture to form myotubes, the cell membranes undergo coordinate series of complex changes. These include the appearance of certain hormone receptors and a dramatic alteration in membrane gangliosides. Thus, gangliosides in addition to their demonstrated function as toxin receptors may potentially be involved in hormonal regulation and cell differentiation.